Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
To manage increased demand for higher bandwidth and data rates, optical systems are increasingly relying on coherent transmission and detection. Such coherent systems require tunable narrow-linewidth lasers as local oscillators. To achieve the narrow linewidth requirements, external cavity lasers are often utilized. Current tunable laser sources suffer from disadvantages such as high optical loss, high sensitivity to system perturbations, limited wavelength tuning and bulky moving mechanical parts.
The tunability of a wavelength tunable laser is typically provided by a tunable optical filter, which selects the output wavelength for laser operation. U.S. Pat. No. 6,141,361 to Mears et al., entitled “Wavelength selective filter”, suggests a wavelength tunable filter formed from a liquid crystal spatial light modulator and a fixed diffraction grating. Such a system, having both a transmissive or reflective grating and liquid crystal device requires a bulky optical configuration and complexity in efficiently coupling between the optical elements. Further, Mears et al. does not provide capability for controlling phase of the tuned wavelengths.
A second known wavelength tunable laser utilizes a liquid crystal cell to provide a resonant waveguide for selective feedback. One known design using these liquid crystal waveguides is described in A. S. P. Chang et al., “Tunable external cavity laser with a liquid crystal sub-wavelength resonant grating filter as wavelength-selective mirror”, IEEE Photonics Technology Letters, 2007, Vol 19, No. 14. Another known laser design utilizing liquid crystal waveguides is described in US Patent Application Publication 2010/246618 A1 to Sudo et al. and entitled “External resonator-type wavelength tunable laser device”. Devices incorporating liquid crystal waveguides, such as Chang et al. and Sudo et al., establish a resonant waveguide in a liquid crystal material having a sub-wavelength grating structure for supporting a wavelength mode perpendicular to the direction of incidence. The resonant mode supported in the waveguide is reflected back into the laser cavity for oscillation. The setting up of a resonant waveguide adds increased fabrication complexity to the device. Further, in these types of devices the tunability is provided by electrically modifying the effective refractive index of the whole liquid crystal material. This analogue control of wavelength through refractive index variation limits the tuning accuracy and renders the devices highly susceptible to variations in material temperature.
A third known wavelength tunable laser incorporates the use of liquid crystal etalons as tunable filter elements. In these devices, a liquid crystal material is placed in the optical path and the refractive index of the material is controlled to define an etalon which supports certain modes. The tunability of liquid crystal etalons is similar in manner to that of the liquid crystal waveguide devices described above. Accordingly, these devices are also highly susceptible to variations in material temperature. Further, in these types of devices, it is difficult to maintain the finesse of an etalon high over long periods of time, primarily due to temperature instability, and the requirement that the transmissive resonance must be double passed within the optical cavity of the laser.
Recent suggestions for increasing transmission data rates in optical systems propose transmitting multiple carrier signals per channel. Many suggested techniques for achieving this require each channel carrier to be coherent in phase in order to be properly detected and decoded. One particular method used for achieving this coherent, multi-carrier transmission is to spectrally carve pulses out of a continuous wave signal. This method maintains the coherent phase relationship but at the expense of a large loss in optical signal power.
Other methods for producing coherent, multi-carrier data transmission include mode-locking a number of frequency separated signals oscillating in a laser cavity. However, these techniques are limited to low repetition rates as they utilize gain switching modulation and the mode-locked frequencies are limited to those frequencies spatially supported by the laser cavity.
There is a need in the art for improved tunable optical filters and wavelength tunable lasers.